1. Field of the Invention
The instant invention relates to a bioprosthetic cardiac valve for acting in the human blood system. More particularly, the invention relates to a self-supporting cardiac valve prosthesis, the design and structure of which make it perfectly suitable for replacing an explanted natural valve.
2. Description of Prior Art
In early studies, approximately until 1960, the normal cardiac valves were described as membranes carrying out a fully passive function depending on pressure and flow variations. Later, several works disclosed the outstanding active features of valve function. In these studies, the aortic cusps (or aortic valve per se) and the aortic root constituted a functional unit.
These papers described the aortic root from a geometrical viewpoint as a truncated cone, the side walls of which are comprised by the Valsalva sinuses; the upper limit is a circumferential line adjacent to the aortic cusps commissures, called the "sinus ridge" or "aortic ridge". The lower limit is comprised by another circumference, a virtual line, adjacent to the nadirs of the aortic valve cusps (leaflets). The prior art also discloses changes existing in the sinus ridge and in the dimensions of the aortic valve nadires during the cardiac cycle, as well as the stress to which the aortic valve cusps are subjected to during such a cycle. Furthermore, experimental work carried out on animals, including hemodynamic angiographic studies have shown that, in general, at a half of the ventricular systole, the diameter of the basal portion of the aortic valve decreases, while the diameter of the aortic ridge (aortic valve commissural plane) increases.
In humans, the behavior of aortic valves has been studied from two radiological positions through cineangiography (30 degrees right anterior oblique position and 60 degrees left anterior oblique position). These studies have demonstrated that the aortic root movements are changing from diastole to systole, frontal-caudally, dorsum-ventrally and from left to right. Frontal-caudal movement is angiographically expressed by a lowering displacement, and dorsal-ventral movement by a forward displacement. The basal portion of the aortic root (nadir) has a gradual diameter decrease during systole, while the commissural plane increases its diameter, and its maximum value is taken at the half of systole. During diastole, these movements are reversed.
On the other hand, the height of the three Valsalva sinuses does not vary simultaneously in the same direction. The movement is reciprocating: a height increase in the right sinus is accompanied by a decrease in the left sinus and, at a given point in time, they are equal. In addition, during the systole period, the shape of the radiological images of the Valsalva sinuses vary; they pass from a globular shape to a straight configuration, in an alternating manner. Therefore, the general configuration of the aortic root at the end of a diastole adopts a geometrical shape similar to a cylinder and, at the end of the systole, its shape is that of an inverted (base up) truncated cone.
During systole, the movement of the aortic valve nadires would be basically influenced by the contractility of the outlet tract of the left ventricle dependent on the cardiac dynamics. Contrary to expectations, the movement of the aortic nadires during systole would not be absolutely uniform due to anatomic relationships. The nadir of the right cusp accompanies the movements of the ventricle septum. Because both the nadirs of the left cusp, and the non-coronary cusp, are integrated to the fibrous tissue of the anterior mitral leaflet and through it to the central fibrous body of the aortoventricle membrane, little movement is provided.
During ventricular systole, the commissural plane of the aortic valve (sinus ridge) has an expansion movement opposed to that of aortic nadires, which depends on the blood volume of the left ventricle and on aortic pressure (volume-pressure dependent). During the post-extrasystole beat, the mentioned movements are stressed, with a lower diameter of the aortic ring nadires (left ventricular contractility increase) and a higher diameter of the commissural plane (related to both left ventricular stroke volume and to the arterial pressure). The outstanding physiology of both the aortic valve and the aortic root was the main objective which promoted the development of a stent-less aortic valve prosthesis. The aortic valve cusps (homologue and heterologue) are directly implanted in the aortic root of the receiving patient.
Mechanical valve prostheses employing metal or plastic components has created multiple problems such as clot formation and the need for subjecting the patient to a permanent treatment with anticoagulants. Later, stented bioprostheses were introduced, such as that disclosed in U.S. Pat. No. 4,079,468, which comprised a low profile support and a porcine aortic valve. A support frame for cardiac tissue valves is disclosed in U.S. Pat. No. 4,259,753.
Although biological valves prostheses eliminated problems due to rejection and clot formation, they nevertheless caused different problems such as reduction of the cross-section (aortic valvular area) and, consequently, a reduction in blood flow. As a consequence, an increase in pressure gradient between the left ventricle and the aorta takes place, due to the presence of the valve support. Furthermore, the flexibility of aortic commissures decrease in stented biological prostheses resulting in a higher stress than that resulting from their normal function.